State scientists, grappling with an explosion of marijuana-growing in the North Coast, recently studied aerial imagery of a small tributary of the Eel River, spawning grounds for endangered Coho salmon and other threatened fish.

In the remote, 37-square-mile patch of forest, they counted 281 outdoor pot farms and 286 greenhouses, containing an estimated 20,000 plants — mostly fed by water diverted from creeks or a fork of the Eel. The scientists determined the farms were siphoning roughly 18 million gallons from the watershed every year, largely at the time when the salmon most need it.

“That is just one small watershed,” said Scott Bauer, the state scientist in charge of the Coho recovery on the North Coast for the Department. of Fish and Game. “You extrapolate that for all the other tributaries, just of the Eel, and you get a lot of marijuana sucking up a lot of water.… This threatens species we are spending millions of dollars to recover.”

The marijuana boom that came with the sudden rise of medical cannabis in California has wreaked havoc on the fragile habitats of the North Coast and other parts of the state. With little or no oversight, farmers have illegally mowed down timber, graded mountaintops flat for sprawling greenhouses, dispersed poisons and pesticides, drained streams and polluted watersheds.

Because marijuana is unregulated in California and illegal under federal law, most growers still operate in the shadows, and scientists have little hard data on their collective impact. But they are getting ever more ugly snapshots.